Microbiologists have recently discovered that microbial communities, some of which are macroscopic and have been studied for hundreds of years, are highly structured and integrated and may actually undergo development by processes similar to those of higher organisms. Microbial communities form in human organ system (e.g., the mouth) and have a beneficial effect on health. Others promote plant growth and carry out the controlled degradative processes that treat sewage and remediate pollutants in our ecosystems. Most microbial processes upon which the well-being of the biosphere depends, from ruminant digestion to sulfur cycling in "black smokers," is carried out by well integrates microbial communities in which we now see a very high level of structural sophistication and developmental control by signaling mechanisms. While a microbial community may resemble a multicellular organism because it has a primitive circulatory system and a degree of cellular specialization, it differs from these life forms in significant ways. The community is not assembled from a single genome, as is a plant or animal, but from a large number of genomes that are programmed to cooperate in community formation. These complex, self-assembling microbial communities are ubiquitous and are not killed by starvation or limited by reproductive necessities, so perhaps we should not be surprised that they re the most successful inhabitants of all ecosystems on earth, including the most hostile and extreme environments. Now that we know the structural and functional complexity of microbial communities that comprise most of the bacteria that affect our medical, dental, agricultural, environmental, and industrial systems, we can use the new tools provided by the engineering disciplines to examine the mature communities. However, we know almost nothing at all about their development. If we are to understand microbial communities sufficiently to promote their development, or even to manipulate them for our own benefit, we must understand their "embryology" and their "physiology." This colloquium on the organization and function of microbial communities has set itself this exciting, and somewhat daunting, task. We plan to convene this colloquium October 14-16, 2001, in San Diego. The colloquium will bring together scientists with broad expertise to consider the science of an issue with broad implications. Participants will have expertise in the following scientific disciplines: microbiology, bioinformatics, signaling, and ecology. We will include Mexican scientists working in the relevant scientific disciplines to achieve a balanced, international perspective. Following the colloquium, a report will be developed, in both print and electronic formats, that will be analytical and comprehensive, yet offer practical recommendations for the future.